I wanted to build a timer application in React using functional component and below are the requirements.

The component will display a number initialized to 0 know as counter.

The component will display a Start button below the counter number.

On clicking the Start button the counter will start running. This means the counter number will start incrementing by 1 for every one second.

When the counter is running(incrementing), the Start button will become the Pause button.

On clicking the Pause button, the counter will preserve its value (number) but stops running(incrementing).

The component will also display a Reset button. On clicking the Reset button, the counter will go to its initial value(which is 0 in our case) and stops running(incrementing).

Below is the code that I have implemented, but clearInterval doesn't seems to be working, Also how do i implement Reset Button?

Code:

import React, { useState, useEffect } from "react";

export default function Counter() {
  const [counter, setCounter] = useState(0);
  const [flag, setFlag] = useState(false);
  const [isClicked, setClicked] = useState(false);
  var myInterval;

  function incrementCounter() {
    setClicked(!isClicked);
    if (flag) {
      myInterval = setInterval(
        () => setCounter((counter) => counter + 1),
        1000
      );
      setFlag(false);
    } else {
      console.log("sasdsad");
      clearInterval(myInterval);
    }
  }

  function resetCounter() {
    clearInterval(myInterval);
    setCounter(0);
  }

  useEffect(() => {
    setFlag(true);
  }, []);

  return (
    <div>
      <p>{counter}</p>
      <button onClick={incrementCounter}>
        {isClicked ? "Pause" : "Start"}
      </button>
      <button onClick={resetCounter}>Reset</button>
    </div>
  );
}

Codesandbox link: CodeSandbox

I wanted to build a timer application in React using functional component and below are the requirements.

The component will display a number initialized to 0 know as counter.

The component will display a Start button below the counter number.

On clicking the Start button the counter will start running. This means the counter number will start incrementing by 1 for every one second.

When the counter is running(incrementing), the Start button will become the Pause button.

On clicking the Pause button, the counter will preserve its value (number) but stops running(incrementing).

The component will also display a Reset button. On clicking the Reset button, the counter will go to its initial value(which is 0 in our case) and stops running(incrementing).

Below is the code that I have implemented, but clearInterval doesn't seems to be working, Also how do i implement Reset Button?

Code:

import React, { useState, useEffect } from "react";

export default function Counter() {
  const [counter, setCounter] = useState(0);
  const [flag, setFlag] = useState(false);
  const [isClicked, setClicked] = useState(false);
  var myInterval;

  function incrementCounter() {
    setClicked(!isClicked);
    if (flag) {
      myInterval = setInterval(
        () => setCounter((counter) => counter + 1),
        1000
      );
      setFlag(false);
    } else {
      console.log("sasdsad");
      clearInterval(myInterval);
    }
  }

  function resetCounter() {
    clearInterval(myInterval);
    setCounter(0);
  }

  useEffect(() => {
    setFlag(true);
  }, []);

  return (
    <div>
      <p>{counter}</p>
      <button onClick={incrementCounter}>
        {isClicked ? "Pause" : "Start"}
      </button>
      <button onClick={resetCounter}>Reset</button>
    </div>
  );
}

Codesandbox link: CodeSandbox

• javascript
• reactjs
• setinterval
• react-functional-component
• clearinterval

5 Answers 5

I did a slightly different version that use an extra useEffect that runs on isRunning (changed name from flag) change:

import React, { useState, useEffect, useRef } from "react";

export default function Counter() {
  const [counter, setCounter] = useState(0);
  // Change initial value to `false` if you don't want
  // to have timer running on load
  // Changed `flag` name to more significant name
  const [isRunning, setIsRunning] = useState(false);
  // You don't need 2 variable for this
  //const [isClicked, setClicked] = useState(false);

  // Using `useRef` to store a reference to the interval
  const myInterval = useRef();

  useEffect(() => {
    // You had this line to start timer on load
    // but you can just set the initial state to `true`
    //setFlag(true);
    // Clear time on component dismount
    return () => clearInterval(myInterval.current);
  }, []);

  // useEffect that start/stop interval on flag change
  useEffect(() => {
    if (isRunning) {
      myInterval.current = setInterval(
        () => setCounter((counter) => counter + 1),
        1000
      );
    } else {
      clearInterval(myInterval.current);
      myInterval.current = null;
    }
  }, [isRunning]);

  // Now on click you only change the flag
  function toggleTimer() {
    setIsRunning((isRunning) => !isRunning);
  }

  function resetCounter() {
    clearInterval(myInterval.current);
    myInterval.current = null;
    setCounter(0);
    setIsRunning(false);
  }

  return (
    <div>
      <p>{counter}</p>
      <button onClick={toggleTimer}>{isRunning ? "Pause" : "Start"}</button>
      <button onClick={resetCounter}>Reset</button>
    </div>
  );
}

Demo: https://codesandbox.io/s/dank-night-wwxqz3?file=/src/Counter.js

As a little extra i've made a version that uses a custom hook useTimer. In this way the component code is way cleaner: https://codesandbox.io/s/agitated-curie-nkjf62?file=/src/Counter.js

Use useRef to make the interval as a ref. Then use resetCounter() to clean the interval ref.

const intervalRef = useRef(null)

const incrementCounter = () => {
  intervalRef.current = setInterval(() => {
    setCounter(prevState => prevState + 1)
  }, 1000);
};

const resetCounter = () => {
  clearInterval(intervalRef.current);
  intervalRef.current = null;
};

I'll just leave this here for anyone having the same problem.

in my case, the issue was node setInterval was used instead of window.setInterval.

this is a problem since this returns a type of Node.Timer which is an object instead of number (setInterval ID) for the clearInterval() to work as it needs an argument type of number. so to fix this,

React.useEffect(() => {
 let timeoutId;
 timeoutId = window.setInterval(callback, 100);

 return = () => {
  if(timeoutId) clearInterval(timeoutId)
 }
}, [])

or in class components use componentWillMount()

Between each rendering your variable myInterval value doesn't survive. That's why you need to use the [useRef][1] hook that save the reference of this variable across each rendering.

Besides, you don't need an flag function, as you have all information with the myClicked variable

Here is a modification of your code with those modifications. Don't hesitate if you have any question.

import React, { useState, useEffect, useRef } from "react";

export default function Counter() {
  const [counter, setCounter] = useState(0);
  const [isStarted, setIsStarted] = useState(false);
  const myInterval = useRef();

  function start() {
    setIsStarted(true);
      myInterval.current = setInterval(() => setCounter((counter) => counter + 1), 100);
      100;
    } 

  function pause() {
    setIsStarted(false);
    clearInterval(myInterval.current);
  }

  function resetCounter() {
    clearInterval(myInterval.current);
    setCounter(0);
  }

  return (
    <div>
      <p>{counter}</p>
      {!isStarted ? 
      <button onClick={start}>
        Start
      </button> 
      :
      <button onClick={pause}>
        Pause
      </button> 
    }
      
      <button onClick={resetCounter}>Reset</button>
    </div>
  );
}
\\\


  [1]: https://reactjs.org/docs/hooks-reference.html#useref

You have to store myInterval in state. After that when button is clicked and flag is false, you can clear interval (myInterval in state).